Impact of Adherence to Target Volume Contouring Guidelines on Patterns of Failure in Sacral Stereotactic Body Radiotherapy.

Abstract

We previously demonstrated that adherence to sacral stereotactic body radiotherapy (SBRT) clinical target volume (CTV) contouring guidelines is independently associated with reduced risk of local failure (LF). We hypothesized that this benefit results from coverage of involved and adjacent boney sectors at risk of subclinical disease and performed a pattern-of-failure (POF) analysis to evaluate the impact of including or excluding these guideline-recommended sectors in the CTV. Of 209 sacral segments treated with SBRT between 2010-2021, 48 developed magnetic resonance imaging-based LF and were analyzed. Failures were mapped by boney sectors and classified as contoured per guidelines (adherent) or recommended but not included in the treated CTV (nonadherent). Dose-volume histogram metrics in nonadherent sectors with subsequent LF were compared with the treated CTV for that segment. Most metastases developing LF involved S1 (42%, 20/48) and S2 (31%, 15/48), were radioresistant (54%, 26/48) and had baseline extraosseous extension (71%, 34/48). Sixty-nine percent (33/48) of segments with LF were adherent, while 31% (15/48) nonadherent. Among all 209 segments, those treated with a nonadherent CTV had higher rates of failure in the involved (37.1% vs 17.8%; p = 0.021) and guideline-recommended adjacent sectors (22.9% vs 2.3%; p < 0.001) than adherent CTV segments. Among adherent segments, most failures occurred exclusively within boney sectors included in the CTV (91%, 30/33). In contrast, 73% (11/15) of failures among nonadherent segments occurred in boney sectors that were recommended but incompletely contoured or omitted from the CTV. The mean equivalent dose in 2-Gy fractions (EQD2, α/β = 10) was significantly lower at 38.7 Gy [interquartile range (IQR), 30.6-42.2] within those nonadherent sectors with tumor progression as compared to the dose in the corresponding treated CTV at 48.2 Gy (IQR, 46.6-49.4; p < 0.001). Anatomic and dosimetric sector-based POF analyses specific to sacral SBRT supports CTV practice guidelines.